The present invention generally relates to semiconductor devices and more particularly to the method of forming a ferroelectric film and fabrication process of a semiconductor device having a ferroelectric film.
A ferroelectric random access memory (FRAM) is a semiconductor device that has a ferroelectric capacitor. A ferroelectric capacitor is a capacitor having a ferroelectric film sandwiched by a pair of capacitor electrodes, and a ferroelectric random access memory stores information in the ferroelectric capacitor insulation film in the form of spontaneous polarization. Thus, an ferroelectric random access memory functions as a non-volatile memory device.
In view of the fact that reversal of spontaneous polarization can be caused in a ferroelectric capacitor insulation film by mere application of a reversing electric field thereto without injection of a current, a ferroelectric random access memory can be written with information (reversal of spontaneous polarization) with minimum electric power. Further, such reversal of the spontaneous polarization can be conducted with very high speed.
Typically, a ferroelectric capacitor for use in a ferroelectric random access memory uses a layer of a perovskite ferroelectric material such as PZT (Pb(Zr,Ti)O3), SBT (SrBi2Ta2O9), and the like, for the ferroelectric capacitor insulation film, and the ferroelectric capacitor insulation film is formed on a lower electrode formed of a material such as Pt (platinum). The lower electrode, in turn, is connected to a conductive region of a transistor by way of a conductive plug such as W (tungsten) provided in an interlayer insulation film.
In order that the ferroelectric film undergoes the desired reversal of polarization with minimum drive voltage, it is necessary that the crystal grains in the ferroelectric film are aligned in a predetermined crystal orientation. Typically, a crystal orientation of <111> or <001> is used in the case of a PZT film, wherein the crystal orientation of <001> is most preferable in view of the maximum electric performance of the film.
On the other hand, there is a possibility in such a conventional construction that the tungsten plug or platinum lower electrode undergoes oxidation at the time of formation of the ferroelectric capacitor insulation film because of the oxidizing ambient that is used at the time of formation of the ferroelectric film.
In view of the problems noted above, the inventor of the present invention has proposed the use of Ir (iridium) for the material of the electrode of the ferroelectric capacitor used in a ferroelectric random access memory. In the case of using iridium, the problem of increase of resistance at the lower capacitor electrode caused by oxidation of the lower capacitor electrode is minimized because of the fact that iridium oxide.
When using iridium for the lower electrode film, it is preferable to form the ferroelectric film by way of an MOCVD process in view of the controllability of the ferroelectric composition and in view of feasibility of mass production.
At the time of forming a ferroelectric film by way of an MOCVD process, it is necessary to cause a decomposition of metal organic source materials of the ferroelectric film in a reactor such that deposition of a ferroelectric film takes place on the iridium substrate, wherein it should be noted that such a decomposition process requires oxygen.
On the other hand, oxygen thus introduced into a reactor, while causing the necessary decomposition of the metal organic source materials of the ferroelectric film, causes also an unwanted oxidizing reaction on the surface of the iridium film, on which the deposition of the ferroelectric film takes place. As a result of such an oxidizing reaction, the surface of the iridium lower electrode is converted to iridium oxide (IrO2).
When the surface of the iridium lower electrode is completely oxidized and there is formed a film of iridium oxide covering the surface of the Ir electrode, the crystal grains of the ferroelectric film thus grown on the iridium oxide film take various crystal orientations, and the desired <001> orientation is no longer obtained. Thereby, problems such as increase of drive voltage at the time of writing or erasing of information are caused associated with the degradation of the electric performance of the ferroelectric film, which in turn is caused by the uncontrolled alignment of orientation of the crystal grains in the ferroelectric film.
Further, it has been difficult to control the progress of the oxidization reaction of the iridium lower electrode and it has been difficult to control the timing of ferroelectric film formation such that the formation of the ferroelectric film is started before there is caused substantial oxidation in the iridium lower electrode.